The present invention relates to the general field of sensors for measuring the level of material in a container, and more particularly to the means for remotely monitoring the level of sediment that accumulates within stormwater management facilities.
Stormwater management facilities, including detention basins and manufactured treatment devices, are installed to capture suspended solids in stormwater in order to improve water quality. Over time, the suspended solids accumulate in the stormwater management facilities and require cleaning. In the absence of a sensor system for measuring the accumulated sediment level, maintenance personnel must periodically physically access the facilities to view sediment levels.
While a number of sensors are available for measuring the level of liquids or solids in a container, these sensors are not adapted to measuring the level of a heterogeneous sludge or slurry of the kind that often accumulates in stormwater facilities. Adding to the complexity of measuring stormwater sediment levels is the frequent presence of floating debris and/or oil.
The patents in this field includes several float-type sensors for monitoring fluid levels. Examples of these are the Lasher (U.S. Pat. No. 3,691,839) and Philbeck (U.S. Pat. No. 6,530,274). Since these devices depend upon the buoyancy of the float in liquid, they are not adaptable to monitoring the level of a solid or semi-solid sediment or sludge.
A non-buoyant float for monitoring the level of sludge in a septic tank is taught by the Bowman (U.S. Pat. No. 4,715,966). A similar non-buoyant float for monitoring the level of sediment in a stormwater management facility is disclosed by the Vitarelli (U.S. Pat. No. 7,596,999). Such contact-based systems have limited accuracy and life cycles due to their moving parts, and they are not well suited to semi-fluid sludges.
Examples of non-contact liquid level monitoring sensors are described in the Snelling (U.S. Pat. No. 6,615,658) and Collins et al. (U.S. Pat. No. 5,929,337) and the U.S. patent publication of Agam et al. (2010/0126267). Since the Snelling sensor is based on the larger difference in thermal conductivity between liquid and vapor phases, it is not readily adaptable to detecting a liquid/solid or air/solid interface. The Collins patent uses ultrasound propagation to measure void spaces in beverage containers. The Agam publication is directed to an ultrasound sensor for detecting fluid interfaces in propane tanks.
A third type of non-contact liquid level monitoring sensor uses infra-red optical radiation from an LED, as taught by the Mruk et al. (U.S. Pat. No. 7,399,985). A fourth type of non-contact level sensor uses differential capacitance to measure the relative liquid content of a vessel, as described in the Brenner et al. (U.S. Pat. No. 5,973,415).
There are two particular problems encountered in measuring the sediment level in a stormwater management facility: (1) the sediment layer is often mixed with and/or suspended in a layer of residual stormwater, such that the liquid/solid interface is not well defined, and (2) floating solid debris is usually present in the stormwater, further obscuring the liquid/solid boundary. Optimally, a dual-sensor system can be used to avoid false and inaccurate readings based on the foregoing factors. For example, an ultrasound sensor could be combined with a capacitive sensor. Under conditions of a dry sediment layer, the ultrasound reading would accurately reflect the sediment level, but the capacitive data would be more relevant under slurry/sludge type conditions.